DESCRIPTION:(from applicant's abstract) The purpose of this grant is to contribute to our understanding, at a neural level, of how animals make decisions to act and choose between actions, with a particular emphasis on factors such as learning and neuromodulation that control the likelihood of behavioral responses. Utilizing both behavioral and electrophysiological methods, we study these processes in a relatively simple system, the lateral giant escape reaction circuit of crayfish, which is subject to a variety of forms of modulation and of simple learning. For the requested funding period we propose experiments on serotonergic modulation of tendency to escape and on habitation of escape behavior. Serotonin can either facilitate or inhibit escape, depending on an animals previous experience or on the exact way in which serotonin titer changes over time; proposed experiments are aimed at understanding the mechanisms of these dependencies, as well as the natural function of the serotonergic modulation. Both intrinsic changes in synaptic efficacy and modulation by GABA-ergic inhibition contribute to habituation; proposed experiments are concerned to understand interactions that appear to exist between these two factors, which may be related by a long term depression-like mechanism.